This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goals of this study are to rigorously test the short-term safety and efficacy of an IL-1 inhibitor (rilonacept) on the articular and systemic manifestations of systemic juvenile idiopathic arthritis (SJIA). The trial will utilize a novel study design that takes into account the rapid onset of response expected with this biologic agent. Treatment Group A, will receive rilonacept at a dose of 4.4 mg/kg up to a maximum dose of 320 mg on day 0, and then weekly at a dose of 2.2mg/kg up to a maximum dose of 160 mg until week 4 when the loading dose is again given, this time with placebo. After the second loading dose, the drug will be administered weekly for the duration of the six-month study. Treatment Group B will get placebo injections in the same way as in Treatment Group A and then at week 4 will receive the loading dose of rilonacept, followed by weekly injections of rilonacept until the duration of the six month study. The trial will enable translational studies by creating a SJIA tissue repository that will bank DNA, RNA, peripheral blood mononuclear cells, serum and plasma collected at strategic intervals throughout the study